1. Field of Invention
This invention relates to optical fiber systems where light is launched into and propagated in the fundamental mode of graded-index multimode fiber.
Several applications for propagation in the fundamental mode of graded-index multimode fiber arises. These include: fiber optic sensor systems, high data rate communication links, low modal noise links, high security links, etc.
2. Purpose
Launching of light exclusively into the fundamental mode of a graded-index multimode fiber is accomplished with this invention. By inserting a length of a special singlemode fiber between an arbitrary fiber carrying light from a source and the graded-index multimode fiber, the light initially in the arbitrary fiber can be launched exclusively into the fundamental mode of the graded-index multimode fiber. The special singlemode fiber has a modal pattern that matches the fundamental Gaussian modal pattern of the graded-index multimode fiber, which permits launching of the light into only the fundamental mode of the multimode fiber.
3. Prior Art
Several studies of selective excitation of low-order modes of graded-index multimode fiber have been made for various applications. Excitation of one specific mode by use of an optical mask and a shifted grating technique has been demonstrated by F. DuBois, Ph. Emplit, and O. Hugon, xe2x80x9cSelective mode excitation in graded-index multimode fiber by a computer generated optical mask,xe2x80x9d Optics Letters 19, pp. 433-435, 1994.
A theoretical study for reducing modal noise by focusing laser light for spot excitation of low-order modes in graded-index multimode fiber is described by J. Saijonmaa and S. J. Halme, xe2x80x9cReduction of modal noise by using reduced spot excitationxe2x80x9d, Applied Optics 20, pp. 4302-4305, 1981. The number and description of modes excited in parabolic-index multimode fiber as a function of the spot radius of the input Gaussian beam and the spot radius of the Gaussian fundamental mode of the multimode fiber were calculated. The light beam from a laser was focused onto the multimode fiber core with a lens.
Light from an arbitrary singlemode fiber has been used by some investigators to launch light into the lowest-order modes of graded-index multimode fiber for special applications:
Bandwidth-distance product has been reported to be doubled by J. Haas and M. A. Santoro, xe2x80x9cA mode filtering scheme for improvement of the bandwidth-distance product in multimode fiber systemsxe2x80x9d, J. Lightwave Technology 11, pp. 1125-1130, 1993. As the title of the publication suggests, the authors used an arbitrary singlemode fiber to launch low-order modes into graded-index multimode fiber and also used a singlemode fiber segment as a mode filter before detection at the multimode fiber terminal. However, the filter evidently gave rise to serious extraneous noise; the authors noted that movement or twisting of the multimode fiber caused serious problems, requiring the fiber to remain stationary. This requirement makes this system inapplicable for field installation.
A study by C. K. Asawa and H. F. Taylor, xe2x80x9cPropagation of light trapped within a set of lowest-order modes of graded-index multimode fiber undergoing bendingxe2x80x9d, Applied Optics 39, pp. 2029-2037, 2000, noted that the problem observed by Haas and Santoro arises from using an arbitrary singlemode fiber for launching the light. Besides launching light into the fundamental mode, other low-order modes are launched. Then, slight movement or twisting of various portions of the fiber causes relative phase shifts between the light in the different launched modes, resulting in differing modal patterns along the fiber. When the output is filtered with the arbitrary singlemode fiber prior to detection in the Haas"" and Santoro""s system, the varying modal patterns due to the fiber movement and the filtering would introduce serious noise problems. Deployed fiber cables buffeted by environmental conditions would make such a system very difficult to use.
Reduction of modal noise by restricting excitation to the lowest-order modes of graded-index multimode fiber has also been studied experimentally, in which He-Ne laser light is injected into an arbitrary singlemode fiber and then launched into a multimode fiber. This was reported by G. C. Papen and G. M. Murphy, xe2x80x9cModal noise in multimode fibers under restricted launch conditionsxe2x80x9d, J. Lightwave Technology 17, pp. 817-823, 1999. No mode profile-matching study was reported.
A graded-index multimode fiber microbend sensor was studied by D. Donlagic and M. Zavrsnik, xe2x80x9cFiber-optic microbend sensor structurexe2x80x9d, Optics Letters 22, 837-839, 1997. The authors used an arbitrary singlemode fiber to launch light into a very short length of graded-index multimode fiber which was part of the microbend sensor. The authors also required a singlemode fiber to filter the light after the sensor for their detection system. The modes excited in the multimode fiber were not analyzed in the paper.
A U.S. Pat. No. 6,185,346, Feb. 6, 2001, inventors C. K. Asawa, Jane K. Asawa, and Mike H. Asawa, entitled xe2x80x9cPropagation in lowest order modes of multimode graded-index fiber, resulting in very low transmission loss, low modal noise, high data security, and high data rate capabilitiesxe2x80x9d, describes launching into the fundamental mode in claim 2: xe2x80x9cwherein mode profile of said singlemode waveguide is designed to closely match mode profile of said fundamental propagation modexe2x80x9d, and again in claim 11. This present invention is different from the above patent claims: in this present invention a length of special singlemode fiber is inserted between the arbitrary singlemode fiber and the graded-index multimode fiber, in order to provide profile-matching of the mode of the special singlemode fiber and the fundamental mode of the graded-index multimode fiber, so that only the fundamental mode is excited in the graded-index multimode fiber.
(1) Our invention refers to the insertion of a length of special singlemode fiber between an arbitrary singlemode fiber propagating light from a source and the graded-index multimode fiber, where the mode profile of the special singlemode fiber matches the fundamental mode profile of the graded-index multimode fiber. The special fiber, therefore, performs the function of launching the light from the source exclusively into said fundamental mode. The length of special fiber may be in the convenient form of a fiber patchcord, or other means indicated in the claims.
(2) If the arbitrary fiber were used to launch the light into the multimode fiber, there would be no assurance that the light would be launched into only the fundamental mode. More likely, the modes launched into the graded-index multimode fiber will be a set of lowest order mode, including the fundamental mode. This can lead to noisy or unusable measurements in some applications, such as experienced by investigators cited above. The cited investigators evidently did not use the powerful diagnostic tool of the xe2x80x9cnear-field imagingxe2x80x9d, as we have, to determine the modes launched into the graded-index multimode fiber, the spot size of the fundamental mode, and the spot size of the of the singlemode fiber.
(3) The theoretical study of Saijonmaa and Halme, cited above, clearly shows that mismatch of the mode profiles of the singlemode fiber with the fundamental mode of a parabolic-index multimode fiber will launch the light into additional mode besides the fundamental mode of the multimode fiber. Hermite-Gaussian modes were the basis wave functions for the modes of a parabolic-index multimode fiber. If the profiles do not match, precise amounts of additional modes launched can be calculated. Their theoretical expression shows that, if the profiles match, light is launched into only the Gaussian fundamental mode of the multimode fiber. Our calculations, which we performed independently, show the same results.